The present invention relates generally to implantable medical devices, and more particularly to an implantable device for detecting the position and movement of the patient. Such devices according to the invention are useful for influencing a patient's bodily functions in conjunction, for example, with cardiac pacemakers, defibrillators, drug infusion pumps, and the like.
A survey of "therapy with rate adaptive cardiac pacemakers" is presented in the article of the same name by E. Alt. et al., published in the journal Herz/Kreislauf 18, No. 11/86, pp. 556 to 564. One section of this article, beginning on p. 560, is devoted to rate adaptive cardiac pacemakers in which the pacing rate is controlled with reference to the measurement of physical activity. An activity sensor consisting of a piezoelectric acceleration sensor located within the pacemaker is provided for detecting the jolts, accelerations and oscillations experienced by the patient in whom the pacemaker is implanted, during physical exercise. The pacing rate is then adaptively regulated in accordance with the intensity of the vibrations that occur and are transmitted throughout the body. Such a cardiac pacemaker is capable of rapid response to commencement of exercise by the pacemaker patient, but it has various disadvantages, such as the inability to maintain adequate continuing increase of pacing rate with ongoing further increase in exercise, and also in distinguishing and appropriately responding to different physical activities having essentially the same intensity of stress.
A further problem in adapting the pacing rate in a physiologically appropriate manner for the patient involves the adequacy of the cardiac pacemaker's rate response when the patient is at rest or undergoes a change in his rest behavior. For example, the fact that a patient's metabolism slows down during sleep is essentially disregarded in the operation of known activity-controlled cardiac pacemakers, because such pacemakers typically have been designed merely to provide a step heart rate increase in response to stress without regard to the nature and extent of the stress. On the other hand, cardiac pacemakers which utilize known representative metabolic parameters to control the pacing rate, that is, parameters such as breathing rate, blood oxygen saturation and the like, have the disadvantage of requiring that the applicable parameter be measured by separate sensors which are outside the confines of the cardiac pacemaker. Also, the latter pacemakers tend to react more slowly to the patient's need for an increase of heart rate with onset of exercise, than do the activity-controlled pacemakers.
Furthermore, such pacemakers are incapable of duplicating the orthostasis reaction of a healthy person in the patient in which the pacemaker is implanted. This type of reaction occurs in a healthy person upon standing from a position of rest, whether lying or sitting, whereupon the individual experiences a rapid compensatory increase in heart rate. When the individual is in a lying position, the venous blood readily returns to the heart from the lower extremities. However, when the person stands, the blood flow from the extremities is slowed because of gravity. This leads to a reduction in the stroke volume of the heart. In a healthy person the reaction is compensated by a reflex increase in the heart rate, but that is not the case with most cardiac patients, and there is no corresponding compensatory effect with known cardiac pacemakers.
In any event, some patients experience a dramatic drop in blood pressure when they change positions, which cannot be overcome merely by use of an implanted cardiac pacemaker. In such instances, it is customary to administer medications to the patient to support the proper blood circulation. It is desirable to limit the administration of such medicinal preparations as much as practicable, and for this and other reasons, portable or implantable drug infusion pumps have been developed that dispense medications slowly in measured concentrations and doses.
As previously noted, a patient's physical position has a considerable influence on the stroke volume of the heart. Accordingly, it may be difficult to determine and regulate the pacing rate merely by reference to measurement of the stroke volume. Therefore, a reliable interpretation of the measured values of the stroke volume would also be desirable.
It is an important object of the present invention to provide new and improved means for controlling and regulating the heart rate of a cardiac pacemaker patient according to the position or physical attitude of the patient relative to an axis determined by gravitational pull.
Another object of the invention is to provide a control mechanism for a cardiac pacemaker by which the pacing rate, and thus the patient's heart rate, may be adjusted according to the specific physical position of the patient while in a state of rest, whereby the heart rate is regulated in a physiologically appropriate manner according to the nature of the rest position and relative changes of the rest position.
A further object of the present invention is to provide a control mechanism for a cardiac pacemaker which is also responsive to acceleration of the patient, so that, for example, when the patient arises from a supine position, the pacing rate may be increased in the same manner as would the heart rate of a healthy person with a normal heart.